The surgical removal of tumors one of the most important medical procedures in the treatment of cancer. Recently, radiopharmaceuticals have been developed which achieve improved specificity and efficiency for labeling malignant tissue and these improvements have generated a resurgence of interest in the use of hand-held, intraoperative nuclear probes to provide in-situ information for the delineation of tumors. While the presently available probes have proven extremely effective for certain types of lesions, clinical tests have shown that in many situations, localization of the malignancy is very difficult to achieve using only the point by point data available from the current probes. Based on the experiences of our many collaborators, we have concluded that the usefulness of nuclear surgical probes would be dramatically improved by the addition of imaging capability. Such an improvement would allow the surgeon to visualize both the tumor itself and the area immediately surrounding it and to use his knowledge of anatomy to more easily explore the extent of the malignancy. We therefore propose to carry out the research necessary to develop a compact nuclear imaging surgical probe for use in the delineation of lesions. In Phase I of the program, we shall design the detector and construct a fully functional laboratory prototype and test its capabilities using phantoms.